The proposed research will result in the design of a small, reliable aerosol sampling and sizing device utilizing the impaction mechanism. It will classify an aerosol according to its aerodynamic equivalent diameter which best describes the behavior of the aerosol in the respiratory tract of animals and humans. The design proposed will eliminate the known errors associated with currently available impactors where severe shifts in particle size distribution do occur due to particle bounce. The relatively low flow rate of no more than 5 liters/min., makes the impactor a desirable sampling instrument in inhalation toxicology where flows through sampling devices can reduce exposure concentrations due to the addtional influx of dilution air. Low flow rates, which can be obtained from portable, battery powered sampling pumps, together with compactness will make the device a convenient field instrument. The detailed calibration procedure with monodisperse aerosols and well characterized poly-disperse aerosols will ensure reliable results during application in the laboratory and under field conditions.